linksToCrawl: Records describing outgoing links from the crawled resources. Should be connected to the same bucket as the input slot.

crawledRecords: Records describing crawled resources. For resources of mimetype text/html the records have the content attached. For other resources, use a webFetcher worker later in the workflow to get the content.

Configuring a proxy

You can configure the proxy the web crawler should use by defining the proxy in the configuration file (see above). E.g. to set up the web crawler to use a proxy at proxy-host:3128, use the following configuration:

Internal structure (TODO: adapt for new implementation!)

To make it easier to extend and improve the web crawler it is divided internally into components. Each of them is a single OSGi service that handles one part of the crawl functionality and can be exchanged individually to improve a single part of the functionality. The architecture looks like this:

The WebCrawler worker is started with one input bulk that contains records with URLS to crawl. (The exception to this rule is the start of the crawl process where it gets a task without an input bulk, which causes it to generate an input record from the task parameters or configuration in a later version.). Then the components are executed like this:

First a VisitedLinksService is asked if this link was already crawled by someone else in this crawl job run. If so, the record is just dropped and no output is produced.

Otherwise, the Fetcher is called to get metadata and content, if the content type of the resource is suitable for link extraction. Else the content will only be fetched in the WebFetcher worker later in the crawl workflow to save IO load.

If the resource could be fetched without problems, the RecordProducer is called to decide if this record should be written to the crawledLinks output bulk. The producer could also modify the records or split them into multiple records, if necessary for the use case.

If the content of the resource was fetched, the LinkExtractor is called to extract outgoing links (e.g. look for <A> tags). It can produce multiple link records containing one absolute outgoing URL each.

If outgoing links were found the LinkFilter is called to remove links that should not be followed (e.g. because they are on a different site) or remove duplicates.

Finally, when all records from an input bulk have been processed, all links visited in this task must be marked as "visited" in the VisitedLinksService.

Outgoing links are separated into multiple bulks to improve scaling: The outgoing links from the initial task that crawls the startUrl will be written to an own bulk each, while outgoing links from later tasks will be written to separate bulks according to the linksPerBulk parameter.
The crawled records are divided into bulks of 100 records at most, but this will usually not have an effect as each incoming link produces one record at most.

Implementation details (TODO: adapt for new implementation!)

ObjectStoreVisitedLinksService (implements VisitedLinksService): Uses the ObjectStoreService to store which links have been visited, similar to the ObjectStoreDeltaService. It uses a configuration file with the same properties in the same configuration directory, but named visitedlinksstore.properties.

SimpleFetcher: Uses a GET request to read the URL. Currently, authentication is not supported. Writes content to attachment httpContent, if the resource is of mimetype text/html and set the following attributes:

httpSize: value from HTTP header Content-Length (-1, if not set), as a Long value.

httpContenttype: value from HTTP header Content-Type, if set.

httpMimetype: mimetype part of HTTP header Content-Type, if set.

httpCharset: charset part of HTTP header Content-Type, if set.

httpLastModified: value from HTTP header Last-Modified, if set, as a DateTime value.

_isCompound: set to true for resources that are identified as extractable compound objects by the running CompoundExtractor service.

SimpleRecordProducer: Set record source and calculates _deltaHash value for DeltaChecker worker (first wins):

if content is attached, calculate a digest.

if httpLastModified attribute is set, use it as the hash.

if httpSize attribute is set, concatenate value of httpMimetype attribute and use it as hash

DefaultLinkFilter: Links are normalized (e.g. fragment parts from URLs ("#...") are removed) and filtered against the specified filter configuration.

Web Fetcher Worker

Worker name: webFetcher

Parameters:

waitBetweenRequests: (opt., see Web Crawler)

filters:

followRedirects: (opt., see Web Crawler)

maxRedirects: (opt., see Web Crawler)

mapping(req., see Web Crawler)

httpUrl(req.) to read the attribute that contains the URL where to fetch the content

httpContent(req.) attachment name where the file content is written to

httpMimetype(opt., see Web Crawler)

httpCharset(opt., see Web Crawler)

httpContenttype(opt., see Web Crawler)

httpLastModified(opt., see Web Crawler)

httpSize(opt., see Web Crawler)

Input slots:

linksToFetch: Records describing crawled resources, with or without the content of the resource.

Output slots:

fetchedLinks: The incoming records with the content of the resource attached.

The fetcher tries to get the content of a web resource identified by attribute httpUrl, if attachment httpContent is not yet set. Like the SimpleFetcher above it does not do redirects or authentication or other fancy stuff to read the resource, but just uses a simple GET request.

For each input record, an input stream to the described web resource is created and fed into the CompoundExtractor service. The produced records are converted to look like records produced by the file crawler. Additional internal attributes that are set:

_deltaHash: computed as in the WebCrawler worker

_compoundRecordId: record ID of top-level compound this element was extracted from

_isCompound: set to true for elements that are compounds themselves.* _compoundPath: sequence of httpUrl attribute values of the compound objects needed to navigate to the compound element.

The crawler attributes httpContenttype, httpMimetype and httpCharset are currently not set by the WebExtractor worker.

If the element is not a compound itself, its content is added as attachment httpContent.

Sample web crawl job

Job definition for crawling from start URL "http://wiki.eclipse.org/SMILA", pushing the imported records to job "indexUpdateJob". An include pattern is defined to make sure that we only crawl URLs from "below" our start URL.